Coincidence type optical rangefinders have been far more popular for civilian use than stereoscopic rangefinders. This is largely due to the fact that coincidence type rangefinders require the use of only one eye to align dual or separated target images. Stereoscopic rangefinders require that both eyes be used and depend on the user's ability to sense and compare target depths. Most persons are able to immediately use coincidence type rangefinders, whereas most people cannot immediately use conventional stereoscopic type rangefinders and many seem unable to develop the ability. The explanation of this inability has a great deal to do with the design of the stereoscopic rangefinder, and as shall be described later, with the user's natural disinclination to accept an artificially introduced reference image, or "range mark", as a believable real target.
Several attempts to provide binoculars with distance measurement functionality have been made. It is convenient to equip binoculars with distance measurement ability for many reasons. Binoculars are generally used for prolonged viewing and are therefore designed for comfort and minimum eyestrain. Magnifications for hand-held binoculars are typically 8 power or less, with binocular entrance window diameters giving exit pupil diameters of 3-5 millimeters. Large exit pupil diameters allow the user to more easily center his eyes on the eyepieces, and ensure that sufficient light will be available to the user's enlarged pupils when viewing in low light or night conditions. Optical rangefinders having these attributes are easier to use and, other factors being equal, are generally more accurate. Additionally, binocular users such as boaters, wildlife observers, air traffic controllers and others are often interested in both viewing magnified images of targets, and in rangefinding the targets. Many binocular and optical rangefinder users pack their equipment into the field and need to minimize the weight of equipment carried. Packing a binocular and rangefinder combination is preferred to packing both binocular and rangefinder separately having larger total size and weight. Unfortunately, current binocular/rangefinder combinations cannot be separated, are significantly heavier than non-rangefinding binoculars and are therefore not acceptable to many binocular users. Field use binoculars are typically worn on a strap around the neck and must be lightweight so as not to be uncomfortable or annoying. The real challenge in the design of the binocular rangefinder then is to produce a design that incorporates comfortable binocular vision, accurate rangefinding, and minimum size and weight.
A few examples of attempts to design suitable binocular rangefinding devices follow. U.S. Pat. No. 4,886,347 entitled "Range-Finding Binocular", to Monroe (1989) describes a stereoscopic binocular rangefinding device. This patent teaches a device which utilizes the principles of stereoscopic rangefinding whereby the user's sense of depth is applied to compare apparent stereo depths of the real sighted target, and an artificial "range-mark" image. The range mark described is a light spot produced from an artificial light source. The light spot is produced in each telescope half of the binocular, and presented to the user's eyes to simulate a target at some depth, or distance. This patent also describes the device's form having "substantially the external appearance and size of a conventional binocular". This invention is of interest as it has true binocular functionality and would therefore presumably provide the required level of eye comfort when viewing targets and rangefinding. The device is however based on traditional stereoscopic rangefinding principles, using an artificial range mark as the reference target for comparing depths of the real sighted target. Stereoscopic rangefinders are well recognized as being difficult for most people to use effectively. During World War 11, when optical rangefinders of all types were used, Donald H. Jacobs, Senior Physicist of the U.S. Naval Observatory, weighing the pros and cons of coincidence and stereoscopic rangefinders, writes: "However, although almost anyone can successfully operate a coincidence instrument, relatively few people can successfully use a stereoscopic device. Some people seem to lose this ability with time". (Fundamentals of Optical Engineering, 1943, page 255.) The fundamental reason this statement is true has much to do with the complex nature of human visual perception, and the inability of many persons to perceive and accept an unnatural "range mark" as a real target. A very similar example is the inability of many persons to "see" 3-D stereoscopic posters that were very popular in North America in 1995, and 1996. Again, a demand is made on a human to use stereoscopic vision in a way that he may find completely unnatural.
Monroe's invention also includes many additional components needed for rangefinding such as mirrors, beamsplitters, a reversible D-C motor, mechanical gearing and power supply. All of these components add weight and size, and reduce the appeal of the device for some binocular users. Monroe has also selected a conventional binocular form, which although convenient, places limits on the practical separation of the binocular entrance windows, or entrance windows. This separation or baselength imposes a fundamental limitation on the accuracy of all triangulation based optical rangefinding systems.
U.S. Pat. No. 2,910,912 entitled "Binocular Telescope With Built-In Rangefinder" to Wohler (1959) also teaches a device in the form of a binocular with distance measurement functionality. This device is perhaps more useful than Monroe's invention for the reason that it is a coincidence type rangefinder. Binocular functionality is degraded however by the inclusion of a beamsplitter plate in each of the telescopes of the binocular. These plates serve to present coincident target images to one of the user's eyes when rangefinding. When using the device for binocular viewing only, the beamsplitter plates need to be removed to avoid presenting confusing dual images in one of the eyepieces. Further, for the device to function adequately when rangefinding, it is necessary, using optical stops, to partially block the entrance window on the side of the binocular presenting dual images to the user's eye. All of these additional optical and mechanical components used to satisfy the requirements of binocular and rangefinding functionality reduce the reliability and increase the weight of the device.